1. Field of the Invention
This invention relates to diaphragm carburetors. More particularly, this invention relates to overcoming problems with the hot restart (vapor lock) of small gasoline engines equipped with diaphragm carburetors.
2. Description of the Prior Art
The hot restart of a gasoline engine, after several minutes the heat "soakback" (equilibration of carburetor temperature with engine temperature), may be very difficult due to the induction of a "vapor lock" condition. Vapor lock is a partial or complete stoppage of fuel flow to the carburetor caused by the formation of gasoline vapor in the fuel system. Partial vapor lock increases the top speed and reduces the power of an engine because the air/fuel mixture is leaned out by the reduced flow of liquid gasoline.
Vapor lock is affected by the following factors:
(1) Temperature and pressure of the gasoline in the fuel system. PA0 (2) Vapor-forming characteristics of the gasoline. PA0 (3) Ability of the fuel system to handle vapor. PA0 (4) Engine operating conditions.
Motor gasolines are composed of liquid hydrocarbons that generally boil between 90.degree. F. and 400.degree. F. under laboratory conditions, e.g., under sea-level atmospheric conditions. Significant amounts of such fuels boil at 135.degree.-140.degree. F. At an altitude of 5,000 feet above sea-level, due to lowered barometric pressure, fuel vaporization occurs at a temperature which is approximately 12.degree. F. lower than sea-level values. Likewise, within the fuel system, fuel system pressure affects vapor formation at a given temperature, increasing amounts of vapor being formed as the pressure is lowered.
Various attempts have been made to compensate for and/or correct the vaporization of fuel in the fuel system.
Coffey, U.S. Pat. No. 2,341,694, discloses a carburetor for internal combustion engines with compensating means for variations in the flow of volatile liquid fuels due to temperature change. In particular, Coffey is concerned with down-draft carburetors which, due to their location with respect to the engine, become heated to such an extent during high speed driving on warm days that the usually available fuels, upon being received in the carburetor constant level chamber, immediately become charged with vapor bubbles. This formation of vapor bubbles causes less fuel to pass through the carburetor metering orifices than if the fuel were cool and not boiling, resulting in a leaning out of the air/fuel mixture being passed to the engine, and frequently, causes the objectionable irregularity of engine operation known as surging. In order to overcome this difficulty, Coffey provides a main carburetor body member having a mixture conduit therethrough. The mixture conduit is provided with a venturi formed restriction and two concentric venturi tubes suspended therein. The outlet of the mixture conduit is controlled by a plate-type throttle valve mounted for rotation with a throttle shaft. Adjacent the mixture conduit and formed as an integral part of the body member is a constant level fuel chamber equipped with a float which acts upon an intake needle valve to maintain a substantially constant level of fuel therein. Attached to the body member is a combination air inlet and fuel chamber cover casting having an air inlet to the mixture conduit formed therein. For control of the air inlet, an unbalanced plate-type choke valve rigidly attached to a rotatable choke shaft is provided. A bimetallic temperature responsive spiral which has its inner end attached to the choke shaft and its outer end attached to a stop is provided for urging the choke toward a closed position with decreasing temperature. Discharging into the venturi tube is a main fuel nozzle which receives fuel from the constant level fuel chamber through a metering orifice and passage. Extending through the orifice is a stepped and tapered metering rod arranged to be moved vertically to vary the net opening of the orifice. A link mechanism is provided for positioning the metering rod in accordance with the position of the throttle valve. In order to compensate for the reduction in flow through the orifice at high temperatures, at which the fuel becomes charged with vapor bubbles, the metering rod is constructed in two sections which are joined by a bimetallic temperature responsive loop. The loop is so constructed as to raise the lower end of the metering rod with increasing temperature and thereby increase the net opening of the orifice for any given position of the throttle valve. Alternatively, two orifices can be provided, the metering rod for one orifice being controlled solely in relation to the position of the throttle valve and the metering rod for the other orifice being controlled solely in response to temperature.
Korte, U.S. Pat. No. 3,186,470, discloses a thermostatic valve for fuel pumps. In particular, conventionally, a fuel system for an internal combustion engine comprises a source of fuel (e.g., a fuel tank), a carburetor for providing the appropriate emulsification of the fuel and its mixture with air, and a pumping means for bringing the fuel from the fuel source to the carburetor. The pumping means, in automobiles, normally is a diaphragm-operated fuel pump located between the fuel tank and the carburetor. When a car is slowed down to a stop, the carburetor fuel bowl fills and the needle valve in the carburetor closes the fuel line from the pump. The pump then builds up pressure of fuel in the line between the pump and the carburetor to a point where the fuel pressure holds the pump out of operation, since the fuel is pumped by spring action rather than a positive action from the pump driving cam of the engine. Thus, upon stopping of the car, fuel between the carburetor and the pump under the spring pressure of the pump will be exposed to the elevated ambient temperature associated with engine heat to an extent that fuel pressure will build up and force fuel through the carburetor into the fuel bowl. The increase in the amount of fuel in the fuel bowl will permit the fuel to run out through the fuel nozzle into the intake manifold of the engine and, upon restarting of the engine, the excessive amount of fuel in the intake manifold results in a difficult starting of the engine. Korte overcomes this problem by providing a thermostatically controlled relief valve on the fuel pump feeding vapors back to the fuel tank and a constant bleed-back to the fuel tank. Under high temperature conditions, the relief valve allows fuel vapors to be passed to the fuel tank, while the constant bleed prevents pressure build-up in the fuel line to the carburetor during shutdown of the engine.
While the above-noted patents provide solutions to the problem of vapor lock for the conventional fuel systems utilized in automobiles, they do not solve the vapor lock problem for diaphragm carburetors typically utilized for two-cycle engines.
In particular, such diaphragm carburetors do not utilize movable stepped and tapered metering rods for control of fuel flow to the fuel nozzles, from a constant level fuel supply, in response to throttle opening; but rather utilize preset metering orifices (e.g., needle valves) which are fed from a fuel chamber which is kept liquid-full by a valve mechanism which is pressure actuated. (The pressure actuation mechanism being disabled with the presence of vapor in the fuel chamber.) Moreover, the diaphragm pumps (actuated by crankcase pressure pulsations) utilized in such diaphragm carburetors would only have their pumping efficiency decreased by provision of a vapor by-pass and/or continuous bleed.
Such diaphragm carburetors are disclosed in Barr, U.S. Pat. No. 3,104,617, and Beck, U.S. Pat. No. 3,269,713.
Additionally, while it is true that the vapor lock condition of such a diaphragm carburetor can be overcome by the use of a manual choke, the two-cycle engines utilizing such carburetors are usually equipped with manual starting mechanisms, e.g., a starting rope. Even utilizing full choke, 10-20 pulls of the starting rope may be necessary of a restart of the engine. This is obviously undesirable in the course of daily use of an engine whih may be repeatedly started and stopped, e.g., a chain saw.
Accordingly, a need continues to exist for a mechanism to avoid the hot restart problems of a vapor locked engine.